According to standard and accepted die making practice, the female die part and the male punch of a complete die are individually machined, heat treated, finished and mounted in a die set. Each complete die has a specified die clearance and draft. The die clearance is the measured space between the mating members of a complete die. For optimum finish of a cut edge of the work part, proper clearance is necessary and is a function of the kind, thickness and temper of the work material. Draft is the amount of taper of the side walls of the female die opening to enable a severed part to be moved through the die without excessive force. Absence of positive draft causes "stacking" or "slug-up" resulting in die failures and excessive press tonnage. Press operators have received personal injury from die failure due to improper die draft. Shattering of punches and splitting of dies to common in the industry and are generally avoided by increased positive draft. However, excess positive draft reduces the economic life of the die. Positive draft occurs when the side walls of the female die opening tape, divergedly from the working face of the female die part where the punch contacts the work material. Negative draft is almost always avoided in the die manufacturing industry.
Once the complete die parts are formed with proper clearance and draft, they are mounted in a die set where it is important that proper registration is achieved. Registration is the relative alignment of the die parts in the die set so that, upon closure of the die set, the die parts will properly come together to maintain the required die clearance.
Due to the foregoing considerations of clearance, draft and registration, die making is a tedious, time-consuming and thus expensive procedure. More inexpensive die making techniques have been developed, but the complete die produced thereby is inferior in quality and generally usable only for short-term runs. For example, a so-called floating punch die is an inexpensive die produced where the punch and the female die part are made from the same piece of tool steel by simply cutting the punch out of the die blank.
Some new, economical die making methods have been developed with the introduction of apparatus for material erosion machining which machine by methods sometimes known as alternative metal removal processes. These include apparatus for electrical discharge machining, electrochemical machining, ultrasonic machining, and lapping. Electrical-discharge machining is based on the principle of erosion of metals by spark discharges. For example, see U.S. Pat. No. 3,604,884 issued Sept. 14, 1971, to Olsson. The spark is a high energy transient electric discharge through the space between two charged electrodes, one of which is a tool and the other a work piece. The discharge occurs when the potential difference between the tool and the work piece is large enough to cause a breakdown in a dielectric fluid medium and to procure an electrically conductive spark channel. As the spacing between the tool and work piece is critical, the feed is generally controlled by servomechanisms.
Apparatus being electrochemical machining process uses a conducting electrolyte which dissolves the reaction products formed on the work piece by electrochemical action and also serves to flush away metallic ions migrating from the work piece. The electrolyte is pumped at high velocities between the forming tool (cathode) and the work piece (anode). A DC power supply maintains very high current densities between the tool and the work piece. For example, see U.S. Pat. No. 3,257,306 to Webb issued June 21, 1966.
An apparatus for ultrasonic machining and lapping employs a process where a tool is given as oscillation, which, in turn, transmits energy to fine abrasive particles that are present between the tool and the work piece. Minute particles of the work piece are eroded away on each stroke. Aluminum oxide, boron carbide, silicon carbide, or diamond particles are typical abrasives used in a slurry, which also carries away the removed debris.
As such apparatus and method are applied to die making, in an electrical-discharge machining apparatus, for example, a die blank is fastened to one shoe of a die set and becomes the work piece. An electrode fashioned in the shape of a punch is fastened to the other shoe and is used to "burn" the die opening in the die blank by bringing an electrode and the bed of the machine together. The die set in use has insulated guide posts. Satisfactory registration can be obtained, however, a negative die draft results, and die clearance is of limited range. Larger clearances and faster removal rates pit the the surface and draw the temper from the tool steel, limiting these methods.